- The rate of twin birth in the United States nearly doubled
between 1980 and 2014,
at which point, almost 3.5% of all US births were twins.
I have a personal interest in twins,
and as you may or may not know,
I have a set of twins myself.
But I also find the process of twinning very interesting.
And so, today we're learning all about twins.
How are they made?
What kinds are there?
What are the risk factors?
And what are some risks and complications
that can come from being pregnant with twins?
(upbeat rhythmic music)
Let's start out by going through some statistics
and risk factors for getting pregnant with twins.
I already went through a few statistics
in the beginning of this video,
the main one being that about 3% of all US births
or 33 in 1,000 births in the US are twins.
That rate varies significantly by where you're located
in the world.
In Japan, the number's closer to one in 1,000,
and in Nigeria, it's closer to 50 in 1,000.
Interestingly, the rates of identical twins
does not really vary across cultures.
So all that variance we see in the numbers
or percentages of twins is accounted for
in non-identical or fraternal twins.
Identical twins don't have a genetic link
or at least not one that we've identified thus far.
We see fraternal twins being very significantly linked
to genetics, and you've probably seen people talk about this
as twins run in my family.
And that can be true,
but only fraternal or non-identical twins run in families.
Fraternal twins tend to run in families
via a process called hyperovulation,
and it's a genetic component
that's not completely teased out,
but we know that it exists
just by going through genealogy tracks.
And interestingly, although it can be passed via the mother
or father, meaning,
if twins run in my dad's family,
I can still get the gene and have hyperovulation.
Or if twins run in my mom's family,
I can get the gene and have hyperovulation
and thus be at an increased risk of having twins.
If twins run in my partner's family,
that doesn't make any difference.
So, only the genes of the person who ovulates are at play
in the risk of twins that are fraternal.
So I think that's interesting, because actually,
as I was going to medical school and stuff,
I thought it could only be passed down
on the maternal side.
Meaning that if there was a family history of twins
like in my dad's side,
I wouldn't be at an increased risk of twins,
but that's not true.
The gene can come via mom or dad.
I don't know if I'm explaining that well at all.
Basically, the family history of twins only matters
if it is in your blood relatives, not your partner's,
because you're the one that would be hyperovulating
and that's how the genetics are passed down.
Which makes sense, right?
I mean, if you think about it,
fraternal twins come from two eggs.
And we'll talk a little bit more about how they happen
in a second, but if it's two eggs,
then this process of creating fraternal twins
is made possible way before a sperm is even introduced
into the picture.
Identical twins are always spontaneous,
meaning they don't have a genetic link
or don't run in families,
at least not to any appreciable amount
that we can detect at this point
in our medical understanding.
Other things that increase someone's risk
of getting pregnant with twins
are utilizing assisted reproductive healthcare,
like IVF, IUI, induced ovulation, age,
the older you are at the time of pregnancy,
the more likely it is that you will conceive twins,
geographic location, as we already discussed,
height and weight, interestingly,
people who are taller and people who are heavier
have an increased risk of getting pregnant with twins,
family history, diet and nutrition,
and also parity or how many deliveries you've had
prior to the one that you are pregnant with,
even when you take out the fact
that most of those people will be older,
even when you control for that,
you're still at an increased risk.
I don't know exactly why this is the case.
It does not make a single bit of sense to me,
but it is associated.
The more deliveries you've already had,
the more likely it is that the next one will be multiples.
(rhythmic music)
I find the physiology of twins, how twins happen,
and the types of twins to be much easier taught
when you have a drawn visual.
Hopefully my audio over here is not too terrible.
I'm not usually set up to film here,
and I'm going to do my best.
As a caveat, I am a doctor, not an artist.
And I'm sorry for that.
First, we're going to talk about the types of twins
and how they are made,
how they come to be.
No, I don't mean how they come to be, as in,
what did their parents have to do to get them,
but what's happening on the inside
of the reproductive organs.
So we have two basic types, monozygotic and dizygotic.
You might know these by their non-scientific names,
identical and fraternal.
Monozygotic twins start as one sperm and one egg,
which becomes fertilized to create one zygote.
Monozygotic, one zygote.
That zygote goes on to split up into two separate embryos.
Identical in that they started from the same set
of genetic material.
With dizygotic or fraternal twins,
you start out with two sperm and two eggs,
which become fertilized separately
and form two separate zygotes.
Those go on to form two embryos,
and they've never shared any genetic material,
at least not any more than a normal sibling set would.
Does this make sense?
Monozygotic, dizygotic, one zygote, two zygotes,
identical, fraternal, all right.
That is the easy part.
Now we move on to the more complex types of twins,
which is what they do on the inside.
(paper rustling)
Clinically, in a pregnancy,
we define twins by what they share.
What you're going to hear me discuss
in this part of the video is chorion,
which is the placenta, and amnion,
which is the amniotic sac that contains the fluid.
Di/di is dichorionic/diamniotic.
Mo/di is monochorionic/diamniotic.
And Mo/mo is monochorionic/monoamniotic.
So, as you can see from these very detailed,
scientifically accurate, and artistic drawings,
di/di twins have separate placentas
and separate amniotic sacs.
Mono/di twins have one shared placenta
and two separate amniotic sacs.
Mono/mono twins have one placenta they share
and one amniotic sac they share.
This is why we see a very unique complication
in twins that are mono/mono.
And that is a complication
called umbilical cord entanglement.
When monozygotic twins form and then they split,
they go in to fall into one of those categories
that I just discussed.
About 70% of them are going to be mono/di,
meaning the twins share a placenta,
but they have separate amniotic sacs.
25% will be di/di,
meaning they each have their own sac
and their own placenta.
5% or less are going to be mono/mono
with one placenta and one shared amniotic sac.
If all of these twins come
from the same process of formation,
what determines which of those categories they fall into?
That is a very ugly uterus.
I apologize.
Ovulation happens out here in the ovary,
and fertilization typically happens in the central part
of the fallopian tube.
After that fertilization happens,
it has to travel down here and implant.
Any time during that period, they can split.
The longer it has been since fertilization,
the more structures they share.
So the point at which that split happens
is the determining factor in what kind of twins you have.
Obviously, they're also going
to be monozygotic and identical.
Day one to three, you have di/di,
and again, not dizygotic, right?
Diamniotic/dichorionic.
They are all monozygotic for this chart.
Day four to eight, mo/di.
Eight to 13, mono/mono.
What happens then if they split after day 13?
A little debated where that number ends.
It might be a little bit closer to like eight or nine.
But regardless, the latest split that can happen
creates another very unique form of twins.
They share body parts.
They are conjoined.
Okay, I hope that that was interesting
and that it accurately and adequately explained this to you.
It is very hot in here,
and I am going to stop filming in front of this light.
Thank you and I will see you back at the desk.
(upbeat music continues)
Now that you know all the twin types, how they are made,
and you've seen all of my beautiful drawings,
let's move on to some risks of a twin pregnancy.
Pregnancy risk is directly correlated
to how much the twins share.
So like we were talking about earlier,
you have this process
from di/di to mono/di to mono/mono to conjoined,
and as you move through that process
with more and more being shared by the fetuses,
then you also see an increasing risk
that goes along with that as far as outcomes
and pregnancy risks.
Vanishing twin is a phenomenon that many of you
have probably heard of,
and this is a process by which, initially,
a pregnancy is noted to have two embryos,
but at the next ultrasound, only one is still visible.
We don't know exactly why this happens,
but it is estimated that up to
maybe one in eight pregnancies start out with two embryos.
But as we discussed earlier,
only about 3% of deliveries, in the US at least, are twins.
So this is a risk of a twin pregnancy,
at least early on.
It's not that they actually disappear or vanish, obviously.
It's just that, at that very early gestation
where we see this happen,
the other twin goes on and develops
into a normal singleton pregnancy
and the tiny embryo that was there initially
just never keeps developing and is reabsorbed by the body.
Preterm birth is the one that we most commonly talk about.
About 30 to 60% of twins are born premature.
Sometimes, this is because we do it iatrogenically,
meaning something is going on
and we make the babies come early
because it's safest for them or for the person
who's pregnant with them.
And other times this is because of an increased risk
of preterm labor or someone's water breaking really early.
Anything that makes the uterus bigger
can cause someone to go into labor earlier,
because your body was really only designed
to carry one baby.
And when there are more than one,
it simply makes your body think
they need to come earlier sometimes.
Another risk of twins is low birth weight
or being born rather small.
And this isn't just small and compounded
from the fact that they're more likely to be premature.
50% of twins are born weighing less than 2.5 kilos,
which is about 5 1/2 pounds,
but we also see an increased incidence
of small for gestational age in twins.
Meaning, if you compare an equal gestational age
for a singleton pregnancy and a twin pregnancy,
the twins are more likely to be smaller.
This causes a myriad of possible complications
that come along with it,
and it's something that needs to be followed closely
and is one of the many reasons
that people who are pregnant with twins
get way more ultrasound scans to monitor the growth
of the babies.
There is also a higher risk of stillbirth
in twin pregnancies,
and this one is very strongly associated
with the more structures shared, the higher the risk.
We see something called twin-to-twin transfusion syndrome,
and this most commonly affects mono/di twins
or twins who share a placenta but have separate sacs.
This is where one of the babies gets too much blood
and nutrition and the other baby does not get enough.
It happens because of the vasculature or vessels
in the placenta being preferentially focused
towards diverting blood to one twin.
And that's called the recipient twin.
The other twin is called the donor twin.
When people hear this,
they automatically think, "Oh, well, the baby
that's losing all the blood is at risk
and the other baby will be fine."
That's not quite true.
Risk is a little bit higher for the baby
who is the donor twin, but the recipient twin is also
at a pretty high risk of complications.
This can be very, very serious,
depending on what gestational age it develops at.
It's something that we watch really, really closely.
Those are some of the risks to the pregnancy
and the outcomes of the pregnancy,
but maternal complications are also significantly higher
in people who are pregnant with twins.
Pretty much, any maternal complication
that you can think of,
any problem that can happen with people who are pregnant
is going to happen more frequently in people
who are pregnant with twins.
This is anything from high blood pressure,
to pre-eclampsia, gestational diabetes,
hyperemesis gravidarum, where someone gets really dehydrated
and malnutrition from not being able
to keep food or drinks down, postpartum complications,
like postpartum hemorrhage.
All of these things are going to be more commonly associated
with twin pregnancies.
One of the most common questions about twins
is about the birth and how should they be born.
How will they be born?
What will the process be?
Planning of delivery is going to be very individual
in twin pregnancies.
There's lots of puzzle pieces to fit together.
What gestational age,
what's the scenario for what's going on,
what's prompting delivery,
what has this person had in their past history
as far as deliveries.
If you look at term twins, so full term twins,
usually considered over 37 weeks,
then in almost all cases
where the babies are both head down,
the recommendation will be to have a vaginal birth.
This is not medical advice.
Obviously, this needs to be a discussion with your doctor,
and there are things that would change that plan.
But as a general rule,
head down babies that are full term
can typically be delivered vaginally.
If baby B, the second baby, is breach,
that is still an option, vaginal delivery,
and just something you need to talk about
with your doctor.
Because most of us are comfortable
with a breach delivery for a second twin,
but that's going to be kinda provider-dependent.
I will say that way more of us are happy
to deliver a second twin vaginally
than a first baby or singleton pregnancy,
because the risks of these things are vastly different.
And that's another topic for another day.
But most of the time,
breach baby B does not mean you are not going to have
a vaginal delivery.
For preterm twins or twins with a significant small
for gestational age component,
this gets a little more complex.
For all of the things that I just discussed,
it applies to both twins who are diamniotic/dichorionic
and monochorionic/diamniotic.
However, if they share a sac,
meaning they are monoamniotic, Cesarean is recommended.
Most of the time, delivery for monoamniotic twins
or twins who share an amniotic sac
is going to be for delivery around 34 weeks.
The reason for this is that we see a very drastic increase
in the risk of stillbirth after that
due to cord entanglement.
Monoamniotic twins always come out
with their cords in a complete mess of a knot.
I will put some pictures on the screen here.
It's very interesting and terrifying
what these babies do in there
with tying their umbilical cord in knots.
Sometimes, it's fine, we deliver them,
they're always a mess of knots like that,
and the babies do fine thanks to something
called Wharton's jelly that has prevented the cords
from getting so compressed that it cuts off blood supply.
But sometimes, that blood supply gets cut off
because they're entangled too tightly.
So that's the reason that monoamniotic twins
are the most high-risk as far as for fetal outcomes.
Most of those patients will be monitored in the hospital
for a very large percentage of the pregnancy
and will be recommended to have a C-section
at about 34 weeks, if not earlier,
based on whatever's going on.
Thanks for being here today, y'all.
I hope that you learned something.
If you don't know, I have a new newsletter
that's going out every Friday called "MDJ in Your Inbox,"
an abridged version
of various trending reproductive health news topics.
You can sign up at the link
in the description box down below.
I will see you next Monday.
(upbeat music continues)